1. Field of the Invention
The present invention relates to semiconductor devices and, more particularly, to light emitting semiconductor devices having excellent carrier confinement capabilities.
2. Description of the Related Art
As communication systems have developed, with both desired and used information transmission rates increasing, more attention has been paid to the development of optical communication systems. As presently contemplated, communication systems will be used from information terminal stations to transmission lines to individual subscriber""s circuits.
To implement and extend such systems, it is necessary to use optical devices such as, for example, light emitting devices, such as laser diodes and light emitting diodes, and photoreceptors, such as PIN photodiodes and avalanche photodiodes. It is desired that these devices be made smaller and less expensive.
For example, although light emitting devices, such as laser diodes, are conventionally accompanied by a cooling device, such as a Peltier element or heat sink to control device temperatures generated by input currents, it is highly desirable for the laser diodes be stably operable even without cooling devices to widely implement light emitting devices in the communication system.
To materialize the stable operation of the laser diodes at practical operation temperatures, it is desirable that these devices have improved device capabilities, such as relatively low threshold current density and low temperature variation of their characteristics. However, such improved capabilities, in particular, temperature characteristics, have not been achieved for conventional GaInPAs/InP laser diodes which contain a GaInPAs active layer with InP cladding layers, both fabricated on an InP substrate, due to the difficulties in achieving a relatively large value of the conduction band discontinuity for these materials.
Several semiconductor laser diodes have been reported to materialize the improved temperature characteristics.
For example, a laser diode containing a GaInNAs active layer disposed on a GaAs substrate is disclosed in Japanese Laid-Open Patent Application No. 6-37355. Therein, it is described that GaInAs layers, having a lattice constant larger than that of GaAs, are added with nitrogen to form GaInNAs layers and to thereby be lattice-matched to GaAs with a resulting reduced lattice constant. As result, it is also disclosed that light emissions at 1.3 xcexcm or 1.5 xcexcm become feasible in these devices.
As another example, calculated results of the energy level line-up are described by Kondo et al in Japanese Journal of Applied Physics, Vol. 35 (1996), pages 1273-5, for a laser diode containing a GaInNAs active layer disposed on a GaAs substrate. It has been suggested in the disclosure that, since the GaInNAs system is lattice-matched to GaAs, a large value of the valence band discontinuity may be acquired by providing cladding layers with AlGaAs rather than the materials which are similarly lattice-matched to GaAs, to thereby be capable of fabricating laser diodes having improved temperature characteristics. In addition, it is also described that the confinement of holes are feasible for the junction structure of a GaInNAs active layer with an AlGaAs layer.
As another example, a laser diode is described by Kondo et al in Japanese Journal of Applied Physics, Vol. 35 (1996), pages 5711-3. As disclosed therein, the laser diode consists of AlGaAs cladding layers with a thickness of about 1.4 xcexcm, GaAs light guide layers with a thickness of about 140 nm, and GaInNAs quantum wells which have a thickness of about 7 nm and the compressive-strained structure Kondo et al demonstrate that the hole confinement is feasible in the GaInNAs layer even with GaAs light guide layers, as long as the GaInNAs layer (1) has a lattice-constant sufficiently larger than that of GaAs and (2) is compressive-strained.
On the other hand, for the GaInNAs active layers which have a lattice constant smaller than that of a GaAs substrate and which have tensile strain, the hole confinement is generally deemed to be unfeasible with light guide layers of GaAs. To realize the hole confinement in the above configuration, accordingly, alloy layers having wider band gap energies such as AlzGa1-zAs (0 less than zxe2x89xa61) or GatIn1-tPuAs1-u (0 less than t less than 1 and 0 less than uxe2x89xa61) alloy layers must be provided contiguous to the GaInNAs active layer. However, both the feasibility and attainment of semiconductor laser diodes having a GaInNAs active layer lattice-matched to GaAs substrate remain elusive.
In addition, there is disclosed in Japanese Laid-Open Patent Application No.7-154023, a laser diode containing an n-GaAs substrate with the following contiguous layers grown thereon, in the order recited: a GaAs buffer layer with 1 xcexcm thickness doped with Si, a GaInP cladding layer of 2 xcexcm thickness doped n-type with Si, a GaInAsP first light guide layer of n-type and 0.2 xcexcm thickness, a GaAs second light guide layer of n-type and 0.1 xcexcm thickness, a GaInAsN (Ga 0.74, N 0.01) active layer of 0.1 xcexcm thickness with a 1.9% strain, a GaAs second light guide layer of p-type and 0.1 xcexcm thickness, a GaInAsP first light guide layer of p-type and xcexcm thickness, a GaInP cladding layer of 0.1 xcexcm thickness doped p-type with Zn, a GaAs contact layer with 1 xcexcm thickness doped with Si, a GaAs current blocking layer doped with Si, and a GaAs contact layer doped with Zn.
Although a GaInAsN active layer of 0.1 xcexcm thickness with a 1.9% strain is described in the disclosure mentioned just above, the crystalline quality of this layer growth does not appear satisfactory because the 0.1 xcexcm thickness of the strained active layer exceeds the critical thickness, thereby resulting in misfit dislocations.
Also, there is described in that disclosure a contact layer doped with Si is disposed between a p-type GaInP cladding layer doped with Zn and a GaAs contact layer doped with Si. Since currents in the laser device flow through the above portion, this averts the currents from the active layer, thereby causing unfavorable results for the laser operation.
In addition, as aforementioned, a laser diode containing a GaInNAs active layer disposed on a GaAs substrate is disclosed in Japanese Laid-Open Patent Application No. 6-37355. However, no detailed description is provided regarding the layer configuration for the laser device. Rather, only the possibility of possible devices but not their feasibility is described.
Yet another laser diode is disclosed in Japanese Journal of Applied Physics, Vol. 35 (1996), pages 5711-3, containing quantum well active layers which are having relatively large compressive strains. However, no description is provided regarding active layers tensile strained or lattice-matched to a GaAs substrate.
Thus, a need exists for an improved light emitting semiconductor device and fabrication process therefor.
Accordingly, it is an object of the present invention to provide an improved light emitting semiconductor devices and fabrication processes therefor, having most, if not all, of the advantages and features of similar employed devices and related processes, while eliminating many of the aforementioned disadvantages of other structures.
In one embodiment of the present invention, a light emitting semiconductor device contains a semiconductor substrate with the following contiguous layers grown thereon, in the order recited: an AlzGa1-zAs (0 less than zxe2x89xa61) or GatIn1-tPuAs1-u (0 less than t less than 1 and 0 less than uxe2x89xa61 lower cladding layer disposed on the semiconductor substrate, having a first conductivity type; a lower spacing layer containing at least one GaAs monolayer; a GaxIn1-xNyAs1-y active layer, having a thickness less than the critical thickness so as not to give rise to misfit dislocations; an upper spacing layer containing at least one GaAs monolayer; an AlzGa1-zAs (0 less than zxe2x89xa61) or GatIn1-tPuAs1-u (0 less than t less than 1 and 0 less than uxe2x89xa61) upper cladding layer, having a second conductivity type, characterized in that the active layer is (1) contiguous to both the lower and upper spacing layers to thereby be disposed on the cladding layers with relative ease, and (2) capable of composing the light emitting device with excellent device characteristics.
In another embodiment, the present invention provides a light emitting semiconductor device which contains the above-mentioned active layer which is composed of a quantum well layer and has a narrow band gap energy, and the lower and upper spacing layers each containing at least one GaAs monolayer; and further provided with lower and upper light guide layers, each of which is composed of AlzGa1-zAs (0 less than zxe2x89xa61) or GatIn1-tPuAs1-u (0 less than t less than 1 and 0 less than uxe2x89xa61) alloy layers, and have band gap energies wider than that of the active layer and narrower than those of the cladding layers. The lower light guide layer is intermediate the lower cladding layer and lower spacing layer, while the upper light guide layer is intermediate the upper cladding layer and upper spacing layer.
By this layer configuration, an improved carrier confinement can be achieved by the light guide layer with a surprisingly wider band gap energy than that of GaAs.
In still another embodiment, a light emitting semiconductor device is provided containing a semiconductor substrate with the following contiguous layers grown thereon, in the order recited: an n-Ga0.5In0.5P lower cladding layer with a thickness of approximately 1.5 xcexcm, a GaAs lower spacing layer with a thickness of approximately 20 nm, a Ga0.9In0.1N0.03As0.97 active layer which is approximately lattice-matched to the semiconductor substrate, has a thickness of about 0.1 xcexcm and a photoluminescence peak wavelength of approximately 1.3 xcexcm, a GaAs upper spacing layer with a thickness of approximately 20 nm, and a p-Ga0.5In0.5P upper cladding layer with a layer thickness of approximately 1.5 xcexcm.
By this structure, light emissions of approximately 1.3 xcexcm can be achieved for the light emitting semiconductor device of the present invention, having improved temperature characteristics over conventional devices composed GaInPAs active layers on an InP substrate.
In yet another embodiment, a light emitting semiconductor device is provided containing a semiconductor substrate with the following contiguous layers grown thereon, in the order recited: Al0.5Ga0.5 As lower cladding layer with a thickness of approximately 1.5 xcexcm, an Al0.2Ga0.8 As lower light guide layer with a thickness of approximately 120 nm, a GaAs lower spacing layer with a thickness of approximately 2 nm, an active layer composed of a Ga0.94In0.06N0.04As0.96 quantum well which is tensile strained, and has a thickness of about 10 nm and a photoluminescence peak wavelength of approximately 1.3 xcexcm; a GaAs upper spacing layer with a thickness of approximately 2 nm, an Al0.2Ga0.8 As upper light guide layer with a thickness of approximately 120 nm, and an p-Al0.5Ga0.5 As upper cladding layer with a layer thickness of approximately 1.5 xcexcm.
With this structure, the light emitting semiconductor device of the present invention with tensile strained quantum well layers affords light emissions of approximately 1.3 xcexcm.